Locomotor activity in morphine-treated rats: effects of and comparisons between cocaine, procaine, and lidocaine.

The effects of cocaine, procaine, and lidocaine on open field and spontaneous (actophotometer) locomotor activities were assessed and compared in rats (1) treated acutely with morphine (single injection), (2) made dependent on morphine (SC pellets), (3) implanted with morphine and withdrawn at the time of peak dependence, and (4) implanted SC with lactose-containing pellets (sham). Cocaine-induced (10 or 30 mg/kg) open field and spontaneous locomotor activities were significantly greater in each of the four groups than those of the corresponding groups administered saline. Procaine (50 or 100 mg/kg) significantly reduced open field locomotor activity in all morphine-treated rats and spontaneous locomotor activity in acute rats. Lidocaine (30 mg/kg) significantly depressed spontaneous locomotor activity in acute rats. Upon comparison of the activities induced by the three local anesthetics, open field locomotor activity of sham-implanted rats was greater following cocaine (10 or 30 mg/kg) than following procaine (50 or 100 mg/kg). Only morphine withdrawn rats manifested greater activity following cocaine (10 mg/kg) than following either procaine (50 mg/kg) or lidocaine (10 mg/kg); activities were equivalent in dependent and acute rats. In contrast, cocaine-induced (30 mg/kg) open field locomotor activity of all morphine-treated rats was greater than either procaine- (100 mg/kg) or lidocaine- (30 mg/kg) induced activities. Spontaneous locomotor activity of all groups except acute morphine was greater following both doses of cocaine than following both doses of either procaine or lidocaine. In acute rats, only cocaine (10 mg/kg, induced greater activity than the other local anesthetics. Thus, stimulation of locomotor activity following cocaine treatment is a pharmacological property unique to cocaine and not shared by either procaine or lidocaine. Further, the data indicate that the methods selected for assessing locomotor activity may not give comparable results.